Taken From November 2009 Edition of Japanese Performance Magazine.
Gobstopper. It’s an appropriate name for a car that silences the bar-room braggers once and for all. This car IS the quickest Impreza of them all. It’s the car to beat in Ten of the Best (winning outright for the past three years), it’s won Time Attack in 2007 and 2008, and it looks like it will do so again in 2009.
Brothers Matt and Olly Clark of Roger Clark Motorsport have spearheaded this unprecedented success. Matt has built and developed the car while Olly uses his considerable driving skills on the track. Gobstopper hasn’t stayed at the top of its game by standing still: it’s constantly changed since its debut back in 2006.
The car started off with the same spec as the famous Team Ice car, also built by RCM, which was the first Impreza to put in a 9sec quarter and really established the Subaru as a serious drag machine. Roll forward three years and Gobstopper might look very similar but it is lighter, lower, more powerful and considerably more sophisticated. And more competitive than ever.
The amazing thing is, almost anyone could build an Impreza just like this, if they have deep enough pockets. Everything from the prepared bodyshell to the choice of pistons is available from RCM.
The starting point for the Gobstopper project was an Impreza Type R Version VI shell, seam-welded and equipped with WRC-type widened arches. A custom T45 steel welded-in rollcage structure was added for safety and stiffness; weight was reduced by using carbon-fibre bumpers and rear spoiler; and the bodywork has had any excess material cut away. The screens are still glass, but the rear one is a lightweight WRC version.
Back in 2006, the car’s purpose was to win drag-based events such as Scooby Shootout and TOTB, so the emphasis was on straight-line rather than circuit performance. The car was lower than standard, but not drastically so, and the suspension was based around simple STi componentry enhanced by rose-jointed pivots and exe-tc dampers and springs.
That drag focus meant high power, initially based on the Team Ice spec featuring a 2.0-litre closed-deck EJ20 block with exotic RCM forged internals and race-spec STi heads. With a GT35-40 turbo and MoTeC M800 management, it could summon 700bhp; with nitrous it would go beyond 800bhp. Gobstopper was ready in time for 2006’s Ten of the Best. ‘Well, it wasn’t quite race-ready,’ says Matt Clark. ‘We had some issues with nitrous backfires in the intake, which split hoses and caused problems, so we didn’t have a brilliant first event. But ever since then, it’s just got better and better.’
One of the car’s key attributes that has remained almost untouched is its engine. While most tuners go for extra capacity, RCM’s car remains at just 2.0 litres. Matt explains: ‘The Impreza engine is much more reliable in 2.0-litre form. It can withstand massive over-revs and serious abuse. You could go larger bore and stroke, but that means more inertia. A lot of the people we race against have 2.4 or 2.5 engines and they might have more torque lower down, but when you’re racing you are never in that band, and the minute you miss a gear the motor grenades itself. It’s better to have a reliable engine. We’ve had some major over-revs from missed gears – I think 12,480rpm is the highest – without major issues. We can run a full season without touching the engine.’
While the basic engine with its Arrow rods, Omega pistons and O-ring head gasket seals has remained, there have been other changes. One of the first was to develop an effective nitrous system. The car had nitrous when first built, but it was only a single-jet system. RCM has now gone to a direct-port system where the nozzles squirt the nitrous almost straight into the cylinders.
Nitrous injection has proved to be a crucial part to the team’s success, delivering an extra 120bhp at the wheels. ‘Nitrous massively cools the intake charge,’ says Matt, ‘and it’s best anywhere with a long straight. The GPS within the MoTeC draws a track map and nitrous can be programmed to come in and out precisely where we want it.’
Even without nitrous, the engine delivers up to 740bhp running 2.3bar boost from the GT4094 turbo. This is a bigger turbo than when the engine was first built but it still gives a decent rev band. ‘When I first built the car I wasn’t a fan of massive turbos,’ says Matt, ‘but they have evolved over the last two years, so now I am. It just works so much better when you try to get a lot of power, provided the turbo is spec'd well for what you are trying to achieve. Running a smaller turbo brings back-pressure issues. We use a twin-scroll turbo which spools up really well. We have developed our own tubular manifold with twin up-pipes and we get really good response.’
When RCM decided to compete in Time Attack for the 2007 season – which it won – it soon became clear that oil surge was a major issue. ‘The datalogging showed massive pressure drops even with the WRC-spec sump. With long corners all the oil runs up the side of the sump,’ says Matt. ‘The dry sump solves the problem and it enables us to move the engine back slightly, which improves weight distribution.’
Also new is an electronic throttle. With the drive-by-wire system, you can run a closed-loop antilag system, so you can adjust the boost available off-throttle. Also, you can program it with the sequential ‘box for flat-throttle shifting and for throttle blipping on the down-change to match the revs to the gear. The MoTeC ECU runs no less than 12 Bosch 700cc injectors – four when off-boost, another four when the turbo spools up, and the final four come in with the nitrous.
Amazingly, some things under the bonnet remain standard. The rad, for example, is stock STi, and so is the power steering. ‘It sounds daft, but we like to keep things standard where we can,’ says Matt.
If it ain’t broke, don’t fix it, seems to be the message here. But one thing that did break in the heat of competition was the clutch input shaft, solved by making that bigger and fitting a larger clutch spline. RCM has a zero-tolerance attitude to breakage. ‘If we have any problem with anything on the car, we always address it and over-engineer it drastically to ensure it will never happen again,’ Matt explains.
Other than that, the RCM-developed Modena transmission has proved to be rock-solid. As for the centre diff, a plate-type LSD is used for drag events but this is too aggressive for circuit use, so an electronically controlled diff is usually fitted, managed by a MoTeC active diff control unit, so that front/rear drive distribution can be driver-controlled.
Although OS Giken’s triple-plate clutches have been reliable, RCM changed to an AP Racing carbon-carbon type for 2009 because it shaves off 10kg. Likewise, the AP brakes were changed to the latest carbon discs to save another 30kg. The carbon brakes need to be 500 degrees C before they give decent braking, and have the potential for greater braking when using slicks, but with road tyres the only benefit is the weight.
More weight has been lost through using carbon-fibre for the doors and front wings. In total, the car now weighs about 80kg less than when first built, but it’s the weight distribution that is arguably as important. Various components have been relocated to get as much weight as possible low down and within the wheelbase of the car. So for 2008, the new dry sump system and its 8-litre holding tank were installed in the left passenger footwell, along with other heavy components such as the nitrous bottle and the fire extinguisher system. The 28-litre fuel tank was moved from the boot to the left side of the car behind the passenger seat. It was a similar story with the battery and washer bottle, while the floor was modified to recess the nitrous bottle and fuel tank lower down.
One thing that has remained constant is the look of the car. ‘I’ve never tried to change the looks,’ says Matt. ‘I always wanted it to roll out at the start of the season and people think it’s the same as it was last year. Everyone we’re racing against in Time Attack uses massive spoilers and diffusers and we want our car to look clean and understated.’
You may notice, however, that it’s now a lot lower. We’re not talking about just winding down the ride height or fitting shorter springs here. RCM altered the entire suspension geometry for the 2007 Time Attack series, which involved fabricating all-new front and rear WRC-style suspension subframes in tubular steel, with new rose-jointed suspension arms, wishbones and even new billet aluminium hub uprights. All that remained from the original set-up were the exe-tc struts.
The suspension alterations necessitated mods to the wheel arches to maintain tyre clearance, and while the bodywork was being cut about, a recess was also cut into the floorpan to accommodate the side-exit exhaust. This was for clearance and to improve aerodynamics, which certainly plays a part (as evidenced by the deeper front splitter and taller rear diffuser compared with the first version). That rear wing must generate some downforce, too.
This car might look old-school with its 10-year-old GC8 shape but there’s no doubting its credentials as a high-tech showpiece. Perhaps the most obvious area to highlight this is the cockpit with its sequential gears and MoTeC display unit with comprehensive datalogging.
When we first built the car, it was intended for TOTB and Scooby Shoootout rather than Time Attack,’ says Matt. ‘In the last two years it has evolved massively to the track car it is now. For Time Attack, you need a car that can go round corners and stop – that’s where the development has been. It hasn’t really gained a lot of horsepower. But it’s still easily possible to do a 9sec quarter with the car.’
That’s what makes Gobstopper such a special car. It isn’t just a drag monster or a tarmac terror. It can do both and do them well, helped, it must be said, by having a driver in Olly Clark with more than his fair share of talent. No doubt some of that is inherited from his rally-champion father, the late Roger Clark.
But what about future developments? ‘We’ve run this car for over three years so maybe it is time for a change,’ says Matt. ‘I always have lots of ideas on how we can improve but I would really like to build a new-shape car. The engine and transmission would swap straight over but we would have to do work on the suspension.’ Whether that happens depends on extra sponsorship, but one thing is certain: Gobstopper looks capable of continuing its race-winning form for some time to come.
As a showpiece for Roger Clark Motorsport, it’s been an unparalleled success. You have to raise your hat to the winning combination of Matt’s car-building skills and Olly’s driving talent. Three times a TOTB winner and on course for being three times a Time Attack Pro Class winner. Results speak louder than any words.